Advanced inspection systems may use either one of two illumination methods: continuous illumination and strobe illumination. In continuous illumination the object and the camera move continuously one with respect to the other while the object is illuminated continuously.
Linear sensors such as CCD linear array or CCD-TDI are used to acquire the image. In the strobe illumination method the object and the camera also move continuously one with respect to the other, but the object is illuminated with short pulses of light. The strobe illumination method enables the use of 2D sensors such as 2D CDD arrays or MOS arrays. The pulse duration of the light source and the scanning speed are adjusted such that the movement of the image with respect to the camera during the light pulse will be at the order of one pixel or less. Common pulsed illumination sources are pulsed lasers and gas discharge flash lamps (i.e. Xe discharge lamp).
Light sources such as discharge lamps, and especially Xe lamps are characterized by long fall time or “tail” as illustrated in FIG. 1 which can go up to 10 μS or more.
Discharge lamps tail also imposes severe limitation on the maximal possible pulse rate (or frame rate) as will be clear from the following example. When using a 1000×1000 pixels camera operating at 500 frames per second, with a single pulse of light per frame—during the frame time (2 mS), the camera and the article move approximately 1000 pixels one with respect to the other to enable acquisition of next frame. In order to ensure image smear of less than 1 pixel during the light pulse, the pulse duration should be less than 2 μS, which is significantly less than what discharge lamps can provide.